Power supply having incrementally related high voltage outputs



Aug. 4,1970

D. s. COCHRAN POWER SUPPLY HAVING INCREMENTALLY RELATED HIGH VOLTAGE OUTPUTS Filed June 6. 1968 INVENTOR DAVID s. COCHRAN AGENT Uni ed State Pat 0.

POWER SUPPLY HAVING INCREMENTALLY RELATED HIGH VOLTAGE OUTPUTS. David S. Cochran, Palo Alto, Calif assignor to-Hewlett- Packard ,Company,-Palo Alto, Calif., a corporation of California 7 Filed June 6, 1968, Ser. No. 735,118 Int. Cl. H02m 7/04 US. Cl. 32115 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION A high voltage component, such as a cathode ray tube, generally is powered by several high voltages which are closely and critically, related to one another. For example, if the cathode electrode of such a tube is driven at 3000 volts, the control grid of the tube should be biased with a slightly more negative predetermined voltage on the order of -3100 volts. Preferably the grid bias voltage is easily and accurately adjustable within a narrow voltage range and is stable with time and temperature. Typical prior art power supplies that provide two incrementally related high voltages employ a power transformer having two secondary windings which drive separate rectifiers and filters. Since both secondary windings must be heavily insulated, the power transformer is generally large and expensive. Also, proper operation is achieved only after carefully equalizing the rectification and filtering attenuation of each circuit. Generally, the output of the bias voltage rectifier and filter is adjusted by a potentiometer. Since the output is on the order of several thousand volts, this arrangement has the disadvantage that adjustments of a few volts are diflicult to make. Additional problems result because the potentiometer is subjected to the full high voltage output. For example, undesirable changes in the resistance of the potentiometer may occur due to variations in temperature and humidity.

SUMMARY OF THE INVENTION The illustrated embodiment of the present invention includes a power transformer having a single high voltage secondary winding which is tapped to provide a source of low voltage. The high voltage is rectified, filtered and applied to an output terminal. The low voltage from the tap of the secondary winding is adjusted by a voltage divider and then rectified and filtered by a voltage multiplier having a capacitively coupled input. The output of the low voltage source is connected to the high voltage output terminal and provides a small voltage increment which is referenced to the high voltage source. The voltage increment is utilizable as a :bias voltage, for example, and can be accurately adjusted by the voltage divider to any desired value within a broad range. Since the voltage divider operates at low voltages, it requires little insulation from ground potential and is not affected by reasonable variations in humidity and temperature.

3,522,516 Patented Aug. 4, 1970 P lCC BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the figure, there is shown a high voltage power supply including a power transformer having primary and secondary windings 11, 13, respectively. The secondary winding 13 is tapped at an intermediate terminal point 15 to define upper andlower sections (as viewed in the drawing) having a turns ratio N :N on the order of 50:1. The number of turns N on the primary winding is selected on the basis of the input voltage E and the desiredoutput voltages. Preferably, E is about 12 volts A.C., and the secondary winding outputvoltage is about 6000 volts peak-to-peak.

The high voltage appearing across the two end terminals .17, 19 of the. secondary winding 13 is half-wave rectified by a diode 21 and thenfiltered by a network including two capacitors 23, 25 and a resistor 27. Terminal 19 is referenced to ground; and a high negative DC. voltage, on the order of 3000 volts, is obtained from output terminal 29.. In the illustrated embodiment of the present invention, output terminal 29 is connected to a cathode ray tube 31 and provides the high negative cathode voltage, ---E however, it is to be noted that the power supply circuit described herein may be also used to drive other high voltage components.

In the case where the upper and lower sections of the secondary winding 13 have the aforementioned turns ratio of 50:1 and the total secondary winding output is 6000 volts peak-to-peak, the output across the tap point terminal 15 and terminal 19 is volts peak-to-peak. This low voltage is adjustable within a predetermined range by a potentiometer 33 which is connected in a voltage divider configuration with a resistor 35.

The movable arm of potentiometer 33 is connected to a low voltage D.C. supply 37, which is a peak-to-peak voltage rectifier or multiplier having an input terminal 39 and positive and negative output terminals 41, 43, respectively. The output terminal 41 is connected directly to the output terminal 29 of the high voltage power supply'. Coupled in parallel between the low voltage terminals 41, 43 are a load resistor 45, a capacitor 47 and two series connected diodes 49, 51. The common junction of diodes 49, 51 is A.C. coupled to the low voltage input terminal 39 through a capacitor 53.

In operation of the voltage multiplier 37, capacitor 53 is charged through diode 49 and the filtering network of the high voltage power supply during the positive halfcycle of the low voltage input to terminal 39 Ge. when tap point terminal 15 is more positive than terminal 19). During the negative half-cycle of the low voltage input, capacitor 47 is charged by current flowing through capacitor 53, diode 51 and the high voltage power supply. The capacitor 47 is charged to a voltage equal to the sum of the positive peak voltage across capacitor 53 and the negative peak voltage input to terminal 39. Thus diodes 49, 51 conduct on alternate one-half cycles to charge capacitors 53, 47, respectively, and the total DC. voltage across terminals 41, 43 is the peak-to-peak value of the AC. voltage input to terminal 39. Load resistor 45 functions to discharge capacitor 47 when the transformer input voltage E is disconnected.

The output voltage of the low voltage D.C. supply 37 is referenced to the high negative DC. voltage at terminal 29 by the aforementioned connection between terminal 29 and terminal 41. Therefore the negative output voltage from terminal 43 will be related to the negative voltage from terminal 29 by a small voltage increment which can be easily adjusted by potentiometer 33. In the illustrated embodiment of the invention, the negative D.C. output voltage from terminal 43 is used to provide the grid bias voltage E for cathode ray tube 31.

I claim:

1. A circuit for producing two high voltage outputs related to one another by an incremental value, said circuit comprising:

means providing a source of high D.C. voltage including: v

a transformer having primary and secondary windings, said secondary winding having two end terminals and an intermediate tap at a predetermined low voltage point near one of said end terminals; and rectifying and filtering means connected to the end terminals of said secondary winding, said rectifying and filtering means having a high voltage output terminal connectable to a load; means providing a source of low D.C. voltage having a magnitude less than the voltage at said high voltage output terminal, said low D.C. voltage source means having a capacitively coupled input terminal, and two opposite polarity output terminals; means connecting the input terminal of said low D.C. voltage source means to the intermediate tap of said secondary winding; and

means connecting the high voltage output terminal of said high D.C. voltage source means to one of said two opposite polarity output terminals of said 10w D.C. voltage source means, thereby to reference the voltage at the other one of said two opposite polarity output terminals to the voltage at said high voltage output terminal.

2. The circuit of claim 1, wherein said low D.C. voltage source means comprises voltage multiplier means including:

a first capacitor connected between said two opposite polarity output terminals;

two diodes coupled in series through a common junction, said two series coupled diodes being connected in parallel with said first capacitor; and

a second capacitor connected in series between the common junction of said two diodes and said input terminal.

3. The circuit of claim 2, said voltage multiplier further including load resistor means connected in parallel with said first capacitor.

4. The circuit of claim 1, said means connecting the input terminal of said low D.C. voltage source means to the intermediate tap of said secondary winding including: adjustable voltage divider means interconnecting said intermediate tap and said nearest one end terminal of said secondary winding.

References Cited UNITED STATES PATENTS 2,744,229 5/ 1956 Rea 32115 2,785,336 3/1957 Konkel et al. 321-15 X 3,132,309 5/ 1964 Constable 321-27 X WILLIAM H. BEHA, JR., Primary Examiner US. Cl. X.R. 321-27 

